Known watercraft of different types and sizes are normally propelled by fixed-pitch propellers, variable-pitch propellers, steering propellers as well as water jet systems. In this case, the mechanical power for propulsion is produced directly by internal combustion engines, for example Otto-cycle engines, diesel engines driven by way of heavy oil or light oil, or gas turbines. Fossil fuels are in this case used here as the energy sources, and are stored in tanks that are carried within the watercraft.
The internal combustion engines on board watercraft are in this case used not only for propulsion, but also for producing electrical power for a large number of electrical loads on board watercraft, for example for auxiliary drives, which in turn convert electrical power to mechanical power, electrical devices for producing heat, cooling and light, electrical devices for media technology and electrical devices for nautical systems. In this case, the internal combustion engines are coupled to electrical machines, which convert the mechanical power that is emitted from the internal combustion engines to electrical power for the electrical loads. The internal combustion engines for propulsion of watercraft in this case produce mechanical power in a range from about 5 kW to about 16 MW, which is made available to the propulsion systems of the watercraft. The power for the electrical power that is required for electrical loads is up to about 15 MVA, depending on the type and size of the watercraft. In this case, depending on the electrical power that is required, two or more on-board network generators, which each have an internal combustion engine, are used for producing electrical power.
The electrical internal combustion engines which operate in the watercraft for propulsion of the watercraft and for producing electrical power for supplying electrical loads via the electrical on-board network of a watercraft in conjunction with on-board network generators, have a number of disadvantages. On the one hand, by burning fossil fuels, the internal combustion engines produce hazardous emissions which endanger the environment and are in general extremely noisy during operation, which is disadvantageous particularly in the case of navy ships since they can thus be detected relatively easily. Furthermore, the fossil fuels (which are carried in tanks) for the internal combustion engines result in the loss of cargo space in the watercraft. In addition, the fossil fuels which are stored in tanks and the on-board network generators means that the internal combustion engines have a heavy weight, which must be moved by the watercraft.
These disadvantages are particularly serious when watercraft are moving in harbor areas, for example when watercraft are entering, leaving or maneuvering in a harbor, since the watercraft result in the harbor area being subjected to concentrated emission loads. The article “Sicherheit auf allen Wegen” [safety in all respects] in etz, Issue 13–14/2000 discloses the use of motors which are operated by means of direct current for propulsion of ships, with these motors being supplied with electrical power by way of fuel cells, for which purpose a DC network is set up within the ship.
Furthermore, DE 197 39 917 A1 discloses a DC supply network for ships, in which two or more AC generators are operated connected in parallel, via controlled rectifiers, such that they are loaded uniformly. In addition, fuel cells can be provided which are connected to the DC supply network.